Added simple colorize options

Hi

We've added a couple of more choices for the colorize option:

- `'level'`, which will only colorize the level (same as `true`)
- `'message'`, which will only colorize the message
- `'all'`, which will colorize both the level and the message

The default behavior is kept, simply set colorize to `true` to colorize the level only, set it to `false` to disable colors.